YouTube: sign up so you can create playlists and favorites. You can subscribe to channels that have your favorite content.
TeacherTube: Works similar to YouTube. Clean and safe. Collection is growing, but "hit and miss" for finding things quickly.
Google Video: Works similar to YouTube.
TED Talks : Theoretical, inspirational and worth listening too. Longer, and designed for an adult audience.
How To Videos : Expert Village......and they have a YouTube Channel at expertvillage
Common Craft: Creators of the beautifully simple "In Plain English" series about RSS, Wikis, Blogs, Social Networking, and more.
PBS for Teachers Lots of fun stuff
Discovery Education : requires subscription, but you can get a free 30 day trial